#22: Autoguiding – Failures and wasted time

31.03.2019

Skywatcher at work

New aspect: Trying autoguiding for the first time

Guiding basics:

During the last sessions I slowly build up the dream of autoguiding my images. There are many advantages of having a guidecam with a guidescope attached to your main scope.

  1. The ability to use the guidescope for advanced polar alignment in phd2. There you use the attached scope to trace movements of stars to determine the polar alignment offset. Phd2 is then able (because it’s clever) to give informations about with direction you need to nudge the scope in order to perfect the alignment.
  2. The ability to use the guidescope with its cam to plate solve. Plate solving is the ability of the PC to count the stars on a given image and compare it to a given database in order to determine the current position of that image in the night sky and give a set of coordinates as an output. Some of you might ask: Why not use the main camera for that? Well, upon now I used the old Olympus E510. This camera can not easiely be controlled by a PC. So in order to use the main imaging cam for plate solving I would have to take a picture, grab the CF-card, transfer the image to the laptop… Long story short: once set up, I don’t want to touch the main camera any more (focus!). Problem is: I need to align the polar scope and the main imaging scope properly in order to use the plate solving abilities sufficiently.
  3. Last but not least: The ability to guide with phd2 to
    1. increase the exposure time! I was up to 60 seconds on lucky sessions, others were more like 50 seconds.
    2. stabilize the framing. Without guiding I had the problem that the image would shift ever so slightly. Even when 60 seconds would give me pin point stars, a night long session would result in a noticeable movement of say half a screen! So I needed to go out and slew back to the original position from time to time. With guiding phd2 would check that for me.
    3. increase the reliability of the system. Without guiding I used say 1/3 of the images max! Some sessions were down to one image out of every 5 frames or so. That was due to bad alignment and other imperfections of my mount. Increasing this quota would give me much more data on a given target!

Guidecam:

  • ToupTek Camera G-1200-KMB Mono Guider
  • 164€
  • CMOS-Chip (AR0130CS)
  • 4,8mm x 3,6mm chip size
  • 3,75 mm pixel size
  • 1280×960 px resolution
  • 8 / 12 bit data depth
  • no active cooling

Guidescope:

  • Omegon Microspeed Guidescope 50mm
  • 129€
  • 200mm focal length
  • 50mm aperture
  • f/4
  • 610g weight

The actual night:

So there are these nights. I set up the rig, connected everything and then… nothing worked.

The ToupTek guidecam had some driver (or hardware?) issues with my laptop. Two scenarios occurred:

  1. The Cam was @ 8bit and then the FPS dropped to one frame every 10 seconds or so and eventually the camera lost connection. Therefore the ST4-impulse guiding the mount worked just fine. But with that frame rate/ lost connection phd2 quit the camera connection ever so often. So guiding was impossible.
  2. The Cam was @ 12/16bit (depending on the program in question). The FPS would rise up to 30frames but the ST4-impulse guiding the mount were lost in nirvana. So guiding was impossible.

The only explanation I have until today is: The camera is wired to work with 12/16 bit. To output 8bit it then downsamples the images ONBOARD and thus dramatically drops the FPS. If the 12/16bit mode is kept then the camera can use the full speed of USB and somehow throws the information about the ST4-guiding impulses out of the USB flow. However. Than could be seen in 8bit mode. Taking an exposure of say 5 seconds would afterwards block the ST4 “channel” for a while until the camera wasn’t busy transmitting the data via USB. Then the ST4 impulse would come through. On my stationary PC I had the same results. ST4 or FPS, never both.

I mailed back and forth with the retailer and finally we decided to swap the camera to ZWO120mini mono.

Additionally I have to say that different forum topics suggest, that this is not a unique problem only occurring to me. Wether this is just an unlucky combination of soft- and hardware or a fundamental problem with the ToupTek G-1200-KMB Mono Guider is just another question.

Conclusion:

I tried the camera in 16bit mode the day before. I managed to get the FPS required and (accidentally turning off the camera) got ST4-control. So I thought I would be prepared. I was mistaken. If something like this happens to you: Nothing works and everything is screwed! You are not alone 🙂 This hobby is by far the most complex and multi-factorial thing I touched. Thousands of variables, interactions, hardware items, software settings, weather influence, camera settings…. What the ***? Keep calm and try again next night. Eventually you will succeed and grow with the experience you gained.

Keep the heads up to the sky!

Greetings

Chris

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